Radiographic imaging, in its simplest expression, is an X-ray beam traversing an object and a detector relating the overall attenuation per ray. The attenuation is derived from a comparison of the same ray with and without the presence of the object. From this conceptual definition, several steps are required to properly construct an image. For instance, the finite size of the X-ray generator, the nature and shape of the filter blocking the very low energy X-ray from the generator, the details of the geometry and characteristics of the detector, and the capacity of the acquisition system are all elements that affect how the actual reconstruction is performed. In the reconstruction, the map of the linear attenuation coefficient (LAC) of the imaged subjects is obtained from the line integrals of the LAC through an inverse Radon transform. The line integrals can be related to the logarithm of the primary intensity of the X-rays passing through the subject. However, the measured X-ray intensity on the detector may include both scattering photons and primary photons. Thus, the images reconstructed from scattering, contaminated intensities may contain some scattering artifacts.
A third-generation CT system can include sparsely distributed fourth-generation, photon-counting detectors. In such a combined system, the fourth-generation detectors collect primary beams through a range of detector fan angles.
Many clinical applications can benefit from spectral CT technology, which can provide improvement in material differentiation and beam hardening correction. Further, semiconductor-based photon-counting detectors are a promising candidate for spectral CT, which is capable of providing better spectral information compared with conventional spectral CT technology (e.g., dual-source, kVp-switching, etc.).
Due to the dead time (˜100 ns), which is determined by the type of semiconductor (e.g. CZT or Cd Te), its thickness and readout circuit, pulse pileup at high X-ray flux (˜108 cps/mm2) can be very severe, and the measured spectral signals can be distorted. The distorted spectral signal can cause artifacts in the reconstruction images. Furthermore, the dead time is not a constant for a given readout circuit due to the location of the pulse formation within the detector cell. However, if the pileup effect can be corrected in the detector model, the image quality can be improved.